The Cascadia Subduction Zone gets its name from the Cascade Mountain Range, and the "subduction zone" part essentially means it's an area where two tectonic plates meet, and one slides beneath another. According to scientific estimates, the CSZ experiences a magnitude 8-9 earthquake every 200-500 years, and as luck would have it, we're about 300 years out from the last one. That means the next event could happen within the next century, or as soon as 50 years from now.
As Kathryn Schulz explains in her fascinating New Yorker piece (bookmark that), FEMA estimates a magnitude 8 or 9 CSZ earthquake -- and the ensuing tsunami -- could result in nearly 13,000 deaths (40,000 total casualties). That insanely high number comes courtesy of the fault's size and location (running alongside Northwestern California, all of Oregon and Washington, and nearly half of Vancouver Island).